Methods of making Z-shielding

ABSTRACT

Methods of building Z-graded radiation shielding and covers. In one aspect, the method includes: providing a substrate surface having about medium Z-grade; plasma spraying a first metal having higher Z-grade than the substrate surface; and infusing a polymer layer to form a laminate. In another aspect, the method includes electro/electroless plating a first metal having higher Z-grade than the substrate surface. In other aspects, the methods include improving an existing electronics enclosure to build a Z-graded radiation shield by applying a temperature controller to at least part of the enclosure and affixing at least one layer of a first metal having higher Z-grade from the enclosure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of U.S. Provisional PatentApplication No. 61/368,361, filed Jul. 28, 2010. This patent applicationalso is related to U.S. Provisional Patent Application Nos. 61/359,955and 61/359,961, filed Jun. 30, 2010.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made in part with Government support under ContractNumbers NN07AA008 and NNL07AM071 awarded by the National Aeronautics andSpace Administration. The Government may have certain rights in thisinvention.

BACKGROUND OF THE INVENTION

Current methods of radiation shielding incorporate thick single layersheet metal using typically one metal or two metals with differentatomic numbers (Z) in separate sheets. Such radiation shielding isneeded for various purposes such as for nuclear reactors or piping forradioactive fluids. Such shielding is also needed for protectiveclothing for nuclear hazardous waste handlers. Such shielding is alsoneeded for various spacecraft, extra-vehicular-activity (EVA) suits, andinstrumentation tools or electronics enclosures. Accordingly, acontinuing need exists for improved methods of making shielding tailoredto reduced thicknesses, weights, and/or specific applications.

BRIEF SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the invention to provide amethod for manufacturing Z-graded radiation shielding with tailoredweights for specific applications.

It is a related object of the invention to provide the ability tomanufacture Z-graded laminates for shielding covers or structures, or toimprove existing electronics enclosures with additional Z-gradedmaterials.

These objects are achieved by the present invention, which providesmethods of building Z-graded radiation shielding and covers. In at leastone embodiment, methods of building a Z-graded laminate are providedthat include the steps of providing a substrate surface having aboutmedium Z-grade; plasma spraying a first metal having higher Z-grade thanthe substrate surface; and infusing a polymer layer to form a laminate.In other embodiments, the methods include providing a substrate surfacehaving about medium Z-grade; electro/electroless plating a first metalhaving higher Z-grade than the substrate surface; and infusing a polymerlayer to form a laminate.

In yet other embodiments, the invention provides methods of improving anexisting electronics enclosure to build a Z-graded radiation shieldhaving the steps of providing an existing electronics enclosure;applying a temperature controller to at least part of the enclosure; andaffixing at least one layer of a first metal having higher Z-grade thanthe enclosure.

Additional objects, embodiments and details of this invention can beobtained from the following detailed description of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Typical radiation shielding has been made by increasing the thickness ofaluminum enclosures to meet radiation reduction requirements. Forexample, methods and programs for engineering design processes forshields controlling radiation doses to targets of interest have beendisclosed in, inter alia, U.S. Patent Application Publication No.2008/0249753 A1, which is entirely herein incorporated by referencethereto. Z-grading is a method of improving radiation shielding by usingmetal layers with varying densities or layers of other materials.Radiation shielding has been traditionally done by increasing aluminumsheet thickness for enclosures to protect electronics. It has been shownthat varying metal densities with different sheets can increaseshielding effectiveness. However, such shielding is typically parasitic,where it increases the weight of the enclosure with only marginalstructural contribution.

The use of fabrics in Z-shielding increases the incorporation offlexible materials and laminate materials into radiation shieldingapplications. For instance, the use of fabrics permits spot shields andcable shields in areas where shielding can improve instrumentationperformance and data transfer. The use of fabrics also permits custommolding and is amenable to having metal(s) affixed using known methods,including plasma spray deposition and/or electro/electroless plating. Insome advantageous embodiments, metals may be layered onto other metalsas well.

In at least one embodiment, a high density metal. such as tungsten ortantalum, or other a high density alloy (like copper—tungsten), isplasma spray coated to fabric, such as carbon fiber or glass fiber. Thena lower density metal, such as copper, is plasma spray coated to fiber.Then a lower density metal, such as titanium or aluminum, can be addedto fiber. Then composite fiber without metal can be added for the lowestZ-grade layer. These varying density fiber layers can be shaped and thenmolded into appropriate geometry with polymeric resin, such as epoxy,cyanoester, or polyimide, in order to make a laminate. These fabrics mayoptionally only be partially infused with polymer or else they may befully laminated into a stiff laminate. They can alternatively be stackedand bent or flexed into a shape and attached to a place needingshielding with varying amounts of polymer as needed.

Direct deposition of tantalum, a refractory metal, onto carbon fiberfabric using radio frequency (RF) plasma spraying has enabled thedevelopment of hybrid metal laminates for reducing high energy electronradiation. Tantalum bulk values and thin film values have been reportedin the literature as 16.6 and 15.6 g/cm³ respectively. See, e.g. Maisseland Glang. Handbook of Thin Film Technology, McGraw Hill Publishing,1970. Tantalum RF plasma spray foil showed a density of 16.02+/−0.02g/cm3, volume resistivity 49+/−6 μohms-cm, and body-centered cubic (bcc)crystal structure. These values are representative of a dense tantalumcoating, important for radiation shielding of energetic electrons. Thecoating process also enables the capability to build functionally gradedlayers with varying atomic number (Z). The incorporation of tantalumcoatings (Z=73) with carbon fiber (Z=6) produces a Z gradient from highto low. Other metals which have also been successfully RF plasma spraycoated on to the carbon fiber fabric include titanium (Z=22) and copper(Z=29). The incorporation of high Z and low Z materials increasesradiation shielding performance as a result of the inherently differentstopping powers of each Z material at the atomic level andcollisional/radiative losses at the nuclear level. The ability to coatcarbon fiber fabric with Z-layers permits processing of radiationshielding with new techniques such as vacuum assisted resin transfermolding (VARTM) in comparison to traditional sheet metal methods. Thesefabrics and resulting consolidated structural composite laminatesdemonstrate utility in non-traditional shielding arrangements, designs,and/or multifunctional structures. In addition, direct tantalum coatingof existing aluminum materials augments radiation shielding to existingspace technologies. Relevant radiation environments includegeostationary earth orbit (GEO) and outer planetary trajectories wherehigh energy electrons dominate. Analytical modeling of the GEO electronradiation environment show gains in radiation shielding when more thanone Z material is layered together. Metal fiber laminates can increasethe radiation shielding effectiveness as a function of weight with abroad range of applications based on composite manufacturing processes.

The incorporation of high Z metal with low Z carbon fiber compositesinto a simple Z-grade builds a tailored radiation shield for high energyelectron radiation. Lower Z metals such as Al, Ti, and Cu have also beensprayed. In some embodiments, preferred features of the new materialsmade by the invention are that layers of RF Plasma sprayed metals can bemanufactured on top of each other. Typical Z-layering approaches requirethe use of metal rod or sheet that must be cut and machined. If morethan one metal is used on top of each other, the materials must beattached by screws or adhesive with individual layers grounded. Varyingthicknesses between sheet layers is difficult because the metal formdoes not allow conformability to the surface. The plasma spray approach,preferably using RF techniques, allows the ability via the sprayingprocess to vary thickness selectively of each layer based on thepositioning in the plasma spray. This coating approach, by increasingshielding in one area and reducing it in other areas, changes the wayshields can be designed and manufactured. The hybrid nature of high Zmetal with carbon fiber laminates offer a game changing opportunity toincrease radiation shielding performance, instrument capability andmission assurance for Space Science and Exploration missions. Radiationis the highest risk to mission assurance for long durationinterplanetary and some of planetary missions for spacecraft andinstruments. The use of structurally efficient radiation shieldingreduces this risk.

Whether metal is affixed to a substrate using plasma spraying or byelectro/electroless plating, the subsequent metal surface may beoptionally polished, finished, sanded, or otherwise treated to obtaindesired thickness, uniformity, and smoothness. One desirable feature ofplasma spraying involves the porous, or semi-permeable nature, of theresulting material, which synergistically allows infusion of polymerthrough laminating processes as known to those of skill in the art.

Plasma spraying and/or electro/electroless plating of metal layers alsoprovides the advantage of allowing the ability to pattern or vary thethickness of different Z-graded layers, as opposed to Z-grade sheets ofmetal which typically only come in fixed thicknesses and uniform size.Such pattern printing may be done through physical masking during thespray process, or by post processing treatments, such as, e.g. etching,scoring, or cracking the material. In some instances, it may bebeneficial to embed functional patterns such as for thermal heat pipes,radiative/conductive/and/or convective cooling piping, channels or otherintegrated functionality, such as would permit piping or channels to beembedded in the shielding to allow circulation of heating or coolingfluids or other thermal conductors as are known to those of skill in theart. Also, with thermal protection in mind, the shielding made bymethods of the invention can also be used as thermal shielding to avoidhigh temperature or low temperature excursions with or without embeddedradiator-like cooling sub-piping or channels. For example, and withoutlimitation, it is believed that tantalum and/or rhenium would makeexcellent thermal shielding material that would provide excellentbenefits when made by the Z-graded methods of the instant invention.

Accordingly, in at least some embodiments, the instant inventive methodsinclude where the shielding cover and structure is selected from thegroup consisting of slabs, vaults, spot enclosures, thermal barriers,and PCI card chassis structures. In some preferred embodiments, theplasma spraying step comprises depositing metal by low-pressure radiofrequency plasma spray technique, such as disclosed and described inU.S. Pat. No. 7,851,062 and references cited therein. The substratesurface can comprise metal, glass or graphite fibers or theirequivalents as known to those skilled in the art. As mentioned above,the metals can comprise a compound selected from the group consisting oftantalum, copper, tungsten, titanium, aluminum, and alloys orcombinations thereof The polymer or resin preferable comprises acompound selected from the group consisting of epoxy, polyimide,cyanoacrylate, and mixtures thereof. In some electronic shieldingapplications, the polymer preferably comprises a conductive filler toprovide increased electro-magnetic-interference resistance. And as alsomentioned above, when a second metal is applied to the shield, it may beapplied over the first metal whereby one or the other metal is used as athermal heat pipe that can be integrated into the shield withoutexpensive post finishing, the use of extra conductive epoxies to bondthe insert, and/or other difficult and expensive post-treatment stepsnecessary to prepare a thermal heat pipe inside a Z-graded shieldmaterial.

Moreover, and without wishing to be bound by any one theory, use, orapplication; it is believed that beneficial uses for shielding made bymethods of the invention include shields for nuclear reactors or pipingfor radioactive fluids, protective clothing for nuclear hazardous wastehandlers or astronauts, and spacecraft instrumentation or electronicenclosures. Shielding may be applicable to electron or gamma rayapplications. Spacecraft applications range from primary shielding ofelectronics and instrument sensors to secondary shielding applicationsfor instruments with specific additional radiation shieldingrequirements. The radiation shielding of transients in CCD arrays is asystemic challenge, where improved shielding methods have significantpayoff in camera resolution and contrast. The combination of high andlow Z materials can be used in layers to take advantage of the differentstopping powers of the materials to increase shielding efficiency as afunction of weight. Accordingly, in some aspects of the invention atleast, the technology is flexible, moldable, and can be made for custom,hard-to-shield locations; it has less weight than traditional radiationshielding material; the shield can be integrated with resins to provideeasy adhesion and a conformal shape; and such flexible shieldingmaterial can be of particular utility in end cap components.

The following examples further illustrate the invention but, of course,should not be construed as in any way limiting its scope.

EXAMPLES

Various samples were prepared in accord with the methods of theinvention.

The Ta Rf Plasma Spray foil was determined to be 16.02 g/cm3. Thelaminate thicknesses were approximated to be 10 mils formed during thelamination process. The Ta/Cyanate composite film thickness were Ta 14mils/30 mils cyanate composite, Ta 15 mils/20 mils cyanate composite, 16mils Ta/10 mils cyanate composite, also 10 mils Ta/70 mils cyanatecomposite. These ratios were compared with aluminum, tantalum, Ta/Al,and cyanate composite. A cyanate ester liquid resin, such as EX1510 byTenCate can be used to infuse the layered materials per the VARTMprocess and cured as known to the art, including such methods asdescribed in U.S. Pat. Nos. 7,595,112 and 7,851,062, which areincorporated herein by reference thereto.

The trend for the electron shielding effectiveness for the samples were:Ta/Al, Ta, Ta/cyanate composite, Al, and cyanate composite.

Aluminum performed best for proton shielding effectiveness: aluminum,cyanate composite, Ta, Ta/cyanate composites.

It was apparent the use of Ta with the cyanate composite contributes toa large increase in mechanical properties as shown in Table 1.

TABLE 1 Modulus and Strength Comparison. Density Materials Z-Numberg/cm³ Modulus Strength Reference Tantalum 73 16.6 186 GPa 650 MPawww.eaglealloys.com Aluminum 13 2.79 68.9 GPa 310 MPa asm.matweb.comCyanate M55J 6 1.60 324 GPa 2303 MPa Hexply 954-3

Table 2 shows that a 15 mil Ta/20 mil cyanate composite had the higheststiffness of the shielding materials. The greater amount of cyanatecomposite will increase stiffness, but reduce electron radiationshielding. It was apparent there was a large gain in thickness reductionand stiffness increase with a Ta/cyanate composite.

TABLE 2 Stiffness Comparison of 0.686 g/cm² materials MaterialsThickness cm (mil) Stiffness Ta 0.0630 (16) 117 MN/m Al 0.254 (100) 175MN/m Cyanate 0.429 (169) 1.39 GN/m Composite Ta/Cyanate (15/20) 110 +164 = 274 MN/m Composite

In another example, a Space 104 conduction cooled aluminum stackableenclosure for space avionics based on the PCI-104 form factor was usedfor a Z-shielding demonstration. Z-shielding was affixedonlto 100 milSpace 104 electronic aluminum endcaps by using high density tantalum onthe endcap top surface, and casting low density engineering polymercoating underneath the endcap. This high-Z, low-Z grading using theendcaps was modeled for radiation performance and shows a reduction ofradiation exposure to approximately 5500 rad/day silicon (ref 105 dayEuropa mission fluence spectra).

It should be noted, that as far as Z-grading metal on other metalmaterials, precautions may be required in some instances to avoidmelting too much or damaging the underlying metal or fabric when addinga high melting metal on top of a lower melting metal layer. Suchprecautions include monitoring or controlling temperature, which may bedone in preferred embodiments by using cold-finger temperaturecontrollers to remove excess heat as known to those of skill in the art.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto he incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

The invention claimed is:
 1. A method of building a Z-graded laminatefor the production of radiation shielding covers or structurescomprising: (a) providing a substrate surface having a Z-grade; (b)forming a layered material by plasma spraying a first metal onto thesubstrate surface, wherein the first metal has a higher Z-grade than thesubstrate surface; and (c) infusing a polymer layer into the layeredmaterial to form the Z-graded laminate.
 2. The method of claim 1,wherein the radiation shielding cover or structure is selected from thegroup consisting of slabs, thermal barriers, vaults, spot enclosures,and PCI card chassis structures.
 3. The method of claim 1, wherein theplasma spraying step comprises depositing the first metal bylow-pressure radio frequency plasma spray technique.
 4. The method ofclaim 1, wherein the substrate surface comprises metal, glass orgraphite fibers.
 5. The method of claim 4, wherein the substrate surfacecomprises glass or graphite fibers.
 6. The method of claim 1, whereinthe first metal comprises a compound selected from the group consistingof tantalum, copper, tungsten, titanium, aluminum, rhenium, and alloysor combinations thereof.
 7. The method of claim 1, further comprisingthe step of plasma spraying a second metal having a higher Z-grade thanthe substrate surface.
 8. The method of claim 7, wherein the secondmetal is different from the first metal, and comprises a compoundselected from the group consisting of tantalum, copper, tungsten,titanium, aluminum, rhenium, and alloys or combinations thereof.
 9. Themethod of claim 7, wherein the first metal is patterned in a shape of athermal heat pipe.
 10. The method of claim 1, wherein the polymer layercomprises a compound selected from the group consisting of epoxy,polyimide, cyanoacrylate, and mixtures thereof.
 11. The method of claim1, wherein the step of infusing is conducted using vacuum assisted resintransfer molding and curing.